Gelator-stabilized crystalline resins

ABSTRACT

A resin composition that is storable at ambient temperatures. The resin composition forms a cured resin when exposed to a curing agent and heated to a curing temperature that is relatively close to ambient temperature. The resin composition includes a resin component that is composed of a liquid part that is made up of one or more liquid thermosetting resins and a solid part that includes particles of one or more solid thermosetting resins. The liquid part further includes a gelation agent that is present in a sufficient amount to maintain the particles in suspension within the liquid part at ambient temperatures. The viscosity of the resin component changes from a high viscosity state to a low viscosity state when the temperature is increased from ambient temperature to the curing temperature. The high viscosity state is substantially more viscous than the low viscosity state.

This application is a divisional of U.S. patent application Ser. No.12/694,302, which was filed on Jan. 27, 2010, which is a divisional ofU.S. patent application Ser. No. 10/586,587, which has issued as U.S.Pat. No. 7,709,803 and which was a 371 of PCT/IB2004/000944.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to thermosetting resins and themany different types of compositions that contain such thermosettingresins. More particularly, the invention involves those types ofthermosetting resins and compositions that are stored at ambienttemperatures and then cured by adding a curing agent and increasing thetemperature of the resin to a curing temperature that is relativelyclose to the storage temperature. The present invention involvesproviding resins that have a relatively high viscosity at ambientstorage temperatures and are suitable for use as adhesives and inprefabricated uncured composites known as prepreg. The resins areconverted to a low viscosity material when heated to curing temperaturesto provide a rapid flow of the resin that may be required for adequatemixing with curing agents and/or penetration into porous bodies such asfiber bundles and fabric.

2. Description of Related Art

Thermosetting resins are used as a principal ingredient in a multitudeof different materials. For example thermosetting resins are widely usedalone or in combination with certain additives to form adhesives.Thermosetting resins are also combined with a wide variety of fibertypes and configurations to form composite materials. Epoxy resins,vinyl ester resins and cyanate ester resins are examples ofthermosetting resins that have been in widespread use for many years.

The curing procedure for thermosetting resins typically involves addingone or more curing agents to the uncured resin to form an activatedresin. The activated resin is then usually heated for a sufficient timeto completely cure the resin. In many situations, it is desirable toprepare the activated resin and then store it for later use. Duringstorage, the activated resins must be kept at temperatures that are wellbelow the curing temperature of the resin in order to avoid prematurecuring. For this reason, many activated thermosetting resins cannot bestored at ambient temperatures. Accordingly, it has been common in thepast to keep such activated resins refrigerated until they are ready tobe used.

Composite materials are used extensively in situations where highstrength and low weight are desired. Composites typically include fibersand a resin formulation as the two principal elements. A wide range offiber types, sizes and orientations have been used in composites. Glass,graphite, carbon, p-aramid, m-aramid, quartz, thermoplastic, boron,ceramic, and natural fibers are common. The fibers can be chopped,stretch broken, randomly oriented, unidirectional in orientation orwoven into fabric. The fibers used in composite materials have diametersthat range from extremely small to relatively large. Although it ispossible to make composites using large diameter fibers, the more commonpractice is to take thousands of fibers having extremely small diametersand form them into individual bundles known as tows. These multi-fibertows are much stronger and more flexible than single fibers having thesame overall dimensions. Filament bundles can have a wide variety ofcross-sectional shapes including ellipsoidal, kidney and pea shapes. Thetows can be woven into fabric in the same manner as conventional yarns.Alternatively, the tows are arranged in parallel to provide aunidirectional fiber orientation or they can be randomly oriented.

Thermosetting resins have been widely used as the resin matrix incomposite materials. There are a number of ways to combine the resinwith the fibers to form the final composite. One approach that has beenused for years is to impregnate the fibers with activated resin andallow the resulting “lay-up” to cure at room temperature. The cure timeis usually reduced substantially by heating the lay-up. This type ofprocess is well suited for use in the field. However, this wet lay-upprocess has a disadvantage in that it is difficult to accurately controlthe amount of resin that is applied to the fibers and ensure that theresin is being uniformly impregnated into the fibers. In addition, theamounts of curing agent and other additives that are added to the resinmay vary between lay-ups.

In order to avoid the above problems, it has been common practice toform prefabricated lay-ups that include fibers, resin and curing agent.These prepregs are made under manufacturing conditions that allow theamount and distribution of resin and curing agent within the fibers tobe carefully controlled. The prepregs are typically refrigerated duringstorage and shipping to prevent premature curing of the resin matrix.The need to refrigerate prepreg presents a number of problems. It isexpensive to store and ship prepreg on a commercial level because largerefrigeration units are required and refrigerated trucks must be used.In addition, the temperature of the prepreg must be continuallymonitored to detect any increase in temperature due to equipment failureor the like. Increases in temperature, even for short periods of time,can adversely affect the shelf life and function of the prepreg andresult in the prepreg being discarded.

One approach to eliminating the need for refrigeration of prepreginvolves placing the resin and curing agent in the prepreg structure sothat they are physically separated from each other. For example, theresin and curing agent can be located on opposite sides of a layer ofwoven fabric to form a prepreg that can be stored indefinitely at roomtemperature as described in U.S. patent application Ser. No. 648,159.When ready for use, the prepreg is heated, usually under pressure, sothat the resin and curing agent flow into the fabric to initiate thecuring process. The basic approach used in these types of systems is tostore the resins and curing agents as separate entities that are insufficiently close proximity to each other so that they can be mixedtogether by heating. This type of approach can also be used forthermosetting adhesives and other applications where the structure ofthe system allows the resins and curing agents to be kept in closeproximity to each other without contact. Such systems typically includea porous body of some type that provides the structure in which theresin and curing agents are located.

There are a number of desirable properties that the resins and curingagents should have in order to be used in the ambient temperaturestorage systems described above. For example, the resin should besufficiently viscous at room temperature so that it does not flow to anyappreciable extent into contact with the curing agent. At the same time,the resin must retain sufficient tackiness and other properties that aredesirable in a prepreg. The resin should be convertible to a relativelylow viscosity material when heated to provide rapid and thorough mixingof the resin and curing agent. The change in resin viscosity shouldoccur at temperatures that are relatively close to room temperature. Forexample, the viscosity change should preferably occur within 10° C. to60° C. above ambient temperatures.

There is a present and continuing need to develop resins that aresuitable for use in prepreg and other systems of the type describedabove that can be stored at ambient temperatures. In addition, there isa present and continuing need to develop prepreg and other systemconfigurations that include resin/curing agent combinations that can bestored at ambient temperatures while still demonstrating the ability toundergo efficient cure at temperatures not significantly higher thanambient temperature. Cure temperatures below 100° C., more preferablybelow 80° C., and most preferably as low as 60° C. are of increasinginterest to resin and/or prepreg converters because the use of thesetemperatures offers significant benefits in terms of energy consumption.Furthermore, as the cure temperature is decreased the processingequipment needed to cure the epoxy resin formulations becomes somewhatsimpler and less expensive. For example, it becomes possible to usetemporary, bespoke curing ovens constructed using inexpensive, butrelatively temperature-sensitive components such as wood andpolyolefinic sheeting. The resins should undergo relatively largereductions in viscosity over relatively small increases in temperatureto provide thorough mixing of the resin and curing agent as well asuniform distribution of the resin throughout the cured structure.

SUMMARY OF THE INVENTION

In accordance with the present invention, resin compositions areprovided that may be used in prepreg and other systems that are storedat ambient temperatures. The resin compositions form a cured resin whenexposed to a curing agent and heated to a curing temperature, preferablyunder pressure, that is higher than ambient temperature. The resincomposition includes a liquid part that is composed of one or moreliquid thermosetting resins and a solid part that is composed ofparticles of one or more solid thermosetting resins wherein the solidpart is dispersed within the liquid part. The particles have a meltingpoint that is above ambient temperature and below the curingtemperature. The liquid part further includes a gelation agent that ispresent in a sufficient amount to form the liquid part into a gel thatis sufficiently gelatinous to maintain the particles in suspensionwithin the liquid part at ambient temperature. The gelation agent has amelting temperature that is below the curing temperature such that theviscosity of the resin changes from a high viscosity state to a lowviscosity state when the temperature of said resin composition isincreased from ambient temperature to the curing temperature. Thisdecrease in viscosity is enabled not only by the thermal breakdown ofthe gelation agent, but also by fusion of the crystalline resinparticles. Unlike higher molecular mass (substantially amorphous) epoxyresins, the melt transition of crystalline resins, as used in thisinvention, occurs over a narrow, predictable, and well definedtemperature range. This invention exploits this characteristic ofcrystalline resins to advantage in controlling the viscosity profile ofthe resin formulation as a function of temperature.

The resins of the present invention are useful in situations where it isdesirable to use relatively small increases above ambient temperature toconvert the resin from a relatively viscous and tacky material to aflowing material that can penetrate porous structures, such as fiberbundles and fabric that contains fiber bundles. The invention coverscompositions that include an uncured resin component, a curativecomponent and a porous body, such as a fiber bundle, a woven fabric or anon-crimped textile. The resin component and curative component areplaced within or on the porous body at separate locations so that theydo not interact to any substantial degree. The viscosity of the resincomponent in the high viscosity state is sufficient to prevent the resincomponent from flowing into or out of the porous body to thereby limitmixing with the curative component. As a feature of the presentinvention, heating of the composition to temperatures only slightlyabove ambient temperatures converts the resin component into the lowviscosity state. The resin component, in the low viscosity state, issufficiently fluid that it flows into contact with the curativecomponent to provide thorough mixing. In addition, the low viscosityform of the resin component flows into and/or out of the porous body toprovide uniform distribution of the resin throughout the curedcomposition.

The above described and many other features and attendant advantages ofthe present invention will become better understood by reference to thefollowing detailed description when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of an exemplary embodiment ofthe present invention where the porous body is a tow or yarn of fiberswherein the resin component is located on the exterior of the tow andthe curing component is located within the interior of the tow.

FIG. 2 is a diagrammatic representation of a prepreg in which fiber towsof the type shown in FIG. 1 are woven to provide a porous body that isimpregnated with uncured resin.

FIG. 3 is a diagrammatic representation of a prepreg in which the porousbody is woven fabric that has be impregnated with the curativecomponent. The uncured resin is provided as a film that is located oneither side of the fabric.

FIGS. 4A-4D are diagrammatic representations of four exemplary prepregconfigurations in accordance with the present invention.

FIG. 5A-5C are diagrammatic representations of three exemplary prepregconfigurations in accordance with the present invention. FIG. 5D is adiagrammatic representation of an adhesive in accordance with thepresent invention wherein the curative component and the resin componentare separated by a fusible barrier.

FIG. 6 is a graphic comparison of the viscosity vs. temperature profilebetween the present invention and other resin systems.

DETAILED DESCRIPTION OF THE INVENTION

Resin compositions in accordance with the present invention are intendfor use in situations where the uncured resin is stored at ambient orroom temperature over extended periods of time. For the purposes of thisspecification, “ambient” or “room” temperature is considered to betemperatures between about 10° C. and about 30° C. More preferably,ambient or room temperature is between about 20° C. and 25° C. Theuncured resin compositions form a cured resin when exposed to a curingagent and heated to a curing temperature that is higher than the ambienttemperature at which the uncured resin is stored. The difference betweenthe ambient storage temperature and the curing temperature rangesbetween 10° C. and 60° C. Curing temperatures of 40° C.-90° C. arepreferred. The resin compositions are in a high viscosity state when theuncured resin is at ambient temperatures. The viscosity of the uncuredresin in the high viscosity state is such that the resin does not flowfreely, but still retains a certain degree of tackiness that is requiredin many applications. The viscosity of the uncured resin in the highviscosity state is preferably between about 20 Pas and 70 Pas.

In the high viscosity state, the uncured resin can be stored inrelatively close proximity to curative components that contain one ormore curing agents. When the temperature of the resin composition isincreased from ambient temperature to the curing temperature, theuncured resin is converted to a low viscosity state. The uncured resinis substantially more viscous in the high viscosity state than in thelow viscosity state. The uncured resin in the low viscosity state isable to flow relatively freely into contact with the curative component.The viscosity of the uncured resin in the low viscosity state ispreferably between about 10 Pas and 0.1 Pas.

The uncured resin includes a liquid part that contains one or moreliquid thermosetting resins and a solid part that includes particles ofone or more solid thermosetting resins that are dispersed within theliquid part. The liquid thermosetting resins may be any of the resinsthat can be stored at ambient temperatures and that can be cured withappropriate curing agents at the curing temperatures set forth above.Resins that are suitable include epoxy resins, vinyl esters, unsaturatedpolyesters, isocyanates, phenolics and cyanate esters. Exemplary resinsinclude epoxy resins such as the glycidyl derivatives of bisphenol A andbisphenol F; the glycidyl derivatives of p-aminophenol andm-aminophenol; monoglycidyl derivatives of aromatic, aliphatic andalicyclic alcohols such as cresyl glycidyl ether, t-butylphenyl glycidylester of neodecanoic acid; polyglycidyl derivatives of polyhydroxycompounds such as glycerol, trimethylolpropane and butanediol;3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate,vinylcyclohexene dioxide, hydrogenated bisphenol A diglycidyl ether,bis(2,3-epoxycyclopentyl)ether, the diglycidyl ester of1,2-cyclohexanedicarboxylic acid, diglycidyl phthalate; aliphatic aminessuch as 1,3-propanediamine, N,N-diethyl 1,3-propanediamine, triethylenetetramine, diethylene triamine, 4,7,10-trioxadecane-1,13-diamine,m-xylylene diamine; aromatic amines such as benzyldimethylamine,benzylamine, 2-4-diethyl toluene diamine and its mixtures,2,4-dithiomethyl toluenediamine, tris(dimethylaminomethyl)phenol;alicyclic and heterocyclic amines such as 1,3-cyclohexane diamine,1,2-cyclohexane diamine, isophorone diamine, p-menthane diamine,methylenebis-(4-cyclohexylamine) and its alkyl substituted derivatives,1,3-bis-(aminomethyl)cyclohexylamine, aminoethyl piperazine,bis(aminopropyl)-piperazine, 1-methylimidazole and other imidazoles;anhydrides such as hexahydrophthalic anhydride, methylhexahydrophthalicanhydride, methyl nadic anhydride; cyanate esters such as the dicyanateesters of bisphenol E, bisphenol A, bisphenol F; vinyl esters such asthe acrylate and methacrylate esters of bisphenol A diglycidyl ether andtheir substituted derivatives and their mixtures with monomeric diluentssuch as styrene, methacrylates and acrylates; diluents and flexibilizersof many classes well known in the art; flame retardants such as thosecontaining phosphorous, including phosphates and phosphonates; catalystssuch as boron trihalides and their amine adducts and mixtures thereofwith diluents and carriers; peroxides and hydroperoxides.

The liquid part also includes a gelation agent that is present in asufficient amount to form the liquid part into a gel that issufficiently gelatinous to maintain the solid particles in suspension.The gelation agent should have a melting point that is below the curingtemperature to which the uncured resin composition is heated. Thegelation agent is preferably a non-polymeric gelator. Any suitablenon-polymeric gelator may be used that is capable of forming the resincomposition into a gel that is able to suspend the solid particles andhas a viscosity that is sufficiently high in the high viscosity state toprevent or minimize flow of the resin composition. A number of suitablegelators that may be used in the resin compositions of the presentinvention are identified in a co-pending United Kingdom Applicationentitled “IMPROVEMENTS IN OR RELATING TO THERMOSETTING RESINCOMPOSITIONS” (Serial No. 0303257.0, filed Feb. 13, 2003). The contentof this United Kingdom patent application is hereby incorporated byreference.

Compounds that may be used as the gelation agent include alkyl ethers ofphenols, especially di-functional and tri-functional phenols. The alkylmoiety of the alkyl ether is preferably within the range of hexyl toocatadecyl. Exemplary compounds include octyl to octadecyl ethers ofcatechol, resorcinol, hydroquinone, 4,4′-biphenol, the naphthalenediols, the antracene diols, the antraquinone diols, pyrogallol,phloroglucinol, stilbene diols and derivatives thereof. The derivativesmay be optionally substituted on either the aromatic ring(s) or thealkyl chain moieties. Examples of suitable substituents for the aromaticring(s) comprise: C₁ to C₄ alkyl; aryl (such as phenyl); C₅ or C₆alicyclic rings; halogen; nitro; alkyl ester; and alkyl amide.Preferably the alkyl moieties of the aforementioned alkyl ester andalkyl amide substituents comprise 1 to 4 carbon atoms. In addition,there may be one or more functional groups on the aromatic ring(s) or onthe alkyl moiety which enable the gelator to react chemically with theone or more constituents of the liquid composition which it is beingused to gel. Especially useful functional groups in this respect arecarboxylic acid, amine, thiol, hydroxyl, oxirane (epoxy), isocyanate,cyanate, allyl and vinyl groups. For example, carboxylicacid-functionalized gelators are capable of reacting with epoxy resinsduring the curing stage, and allyl- and vinyl-functionalized gelatorsare capable of reacting with vinyl ester and unsaturated polyesterresins during the curing stage.

The above-described gelation agents may be prepared by any of themethods known in the literature. For instance, the phenyl ether gelatorswhich find use in this invention may be prepared via the Williamsonsynthesis involving the reaction of a phenate salt with an alkyl halidein the presence of a base in the suitable solvent. Reduction of estersand the reaction of the phenol with the appropriate α-olefin are otherpotential routes. Other reaction conditions may also be employed, forexample, the use of supercritical fluids such as carbon dioxide mayfacilitate the reaction.

The alkyl ureas of aromatic isocyanates are also considered to beparticularly suitable gelation agents. Suitable isocyanates are toluenediisocyanate, 1,4-phenylene diisocyanate,methylenebis(4-phenylisocyanate), xylene diisocyanate,1,8-naphthalenediisocyanate. Preferably, the carbon chain length is inthe range 6 to 18 carbon atoms. The urea compounds may alternatively bemade by taking the alkyl isocyanate and reacting with the relevantaromatic diamine. As such, the preferred route will be determined by theavailability of the amine and isocyanate component.

The quantity of gelation agent that is present in the liquid part can bevaried and will depend upon a number of factors. These factors includethe types of resin or resins used, the amount and size of solidparticles that are to be suspended in the gel and the particulargelation agent. In general, the amount of gelation agent that isrequired to obtain the above identified high and low viscosity statescan be determined by routine experimentation. Typically, the amount ofgelation agent will be in the range of from 1 to 10 percent by weight ofthe liquid part of the resin composition. About 5% gelation agent byweight is preferred.

The solid part of the resin composition is composed of solid particlesthat are sufficiently small that they can be suspended in the liquidpart. Particle sizes that range from 10 mm down to 0.001 mm aresuitable. Particles that are smaller than 5 mm are preferred withparticle sizes of less than 0.5 mm being particularly preferred. Theparticles can be made from any thermosetting polymer or resin that issolid (crystalline) at room temperature and has a melting point that isbelow the curing temperature selected for the particular resincomposition. Exemplary preferred solid particles are made from materialssuch as, crystallized Ruetapox 0158 that is available from Bakelite AG(Duisberg, Germany) and crystallized DER 332 that is available from DowChemical Company (Wilmington, Del.).

Like the gelation agent, the amount of solid particles in the uncuredresin composition will vary depending upon the same type of factors. Anoverriding consideration is that the particles must remain suspended inthe liquid part for relatively long periods of time at ambienttemperatures. The actual amount of solid particles needed to achieve thetarget high and low viscosity states can also be determined by routineexperimentation. Typically, from 0.5 to 10 parts by weight particleswill be added to 1 part by weight of the resin/gelation agent mixture.

The uncured resin compositions are preferably prepared by dissolving thegelation agent in the liquid resin at an elevated temperature.Temperatures on the order of 80° C. to 120° C. are preferred. Once thegelation agent is dissolved, the liquid part is cooled to a temperaturethat is no lower than about 30° C. The solid particles are then addedand thoroughly mixed into the liquid part. The resulting resincomposition is then applied to a porous body or formed into a film orother configuration depending upon the final intended use. Once theresin composition has been manipulated into the appropriateconfiguration or form for storage, it is cooled further to ambienttemperature. At ambient temperature, the resin composition is in thehigh viscosity state and can maintain the solid particles in suspensionfor relatively long storage times. Storage times may be as short as afew hours or as long as a few months or more.

The uncured resins are preferably stored in combination with a curativecomponent that is located in close proximity to the resin, but not inchemical contact therewith. The resin and curative component may simplybe stored side by side with a barrier located between them. However, itis preferred that the resin and curative component be stored incombination with one or more porous bodies that provide a structure onwhich the two components may be stored in close proximity withoutchemical contact. The porous body can be anything that has an exteriorsurface and interior surfaces that are located within the porous body.The preferred porous bodies are those composed of the fibers that areused in making composite materials. Other porous bodies that may be usedinclude open cell foam, honeycomb and thermoplastic scrims.

A single tow of fibers may be considered to be a porous body inaccordance with the present invention. As shown in FIG. 1, the tow offibers shown generally at 10 includes an outer discontinuous circularsurface (represented at 12) that is formed by the outer surfaces of theindividual filaments 14 that are located at the surface of the tow 10.The tow 10 also includes interior filaments 16 that have surfaces whichdefine the interstitial spaces 18 located within the tow 10. Thecurative component is distributed on the surfaces of the interiorfilaments 16 or within the interstitial spaces 18 as represented by the“+'s”. The resin component completely surrounds the tow 10 as shown at20. At room temperature, the resin is in its high viscosity state andremains substantially in place and does not flow into the interstitialspaces. When heated to its low viscosity state, the resin flows into theinterstitial spaces where it mixes with the curative component, fillsthe interstitial spaces and wets all of the dry surfaces of the interiorfilaments. If desired, an optional removable reaction barrier 22 isprovided to keep the resin component from contacting any of the filamentsurfaces. Without the barrier 22, the resin wets the outer surfaces ofthe exterior filaments 14. The removable barrier is preferably made froma thin polymeric film that melts or otherwise is dissolved by the resinat temperatures between the ambient temperature at which the compositionis stored and the curing temperature. Examples of suitable barrier filmmaterials include aqueous film-forming emulsions and dispersions ofpolyethylene; oxidized polyethylene; ethylene copolymers with acrylicand methacrylic esters and acids; polyethylene waxes; carnauba wax andother naturally derived waxes. Thin films of polyethylene,poly(ethylene-co-propylene) and other ethylene copolymers; eitherapplied as hot melts or from solution can also be used.

The curative component that is eventually combined with the resincomponent includes one or more curing agents that may or may not bedissolved or otherwise suspended in a carrier. The curing agent(s) areselected such that they provide curing of the resin component whencombined therewith at temperatures equal to or below the above statedcuring temperatures. The amount of curing agent required to provideadequate curing of the resin component will vary depending upon a numberof factors including the type of resin being cured, the desired curingtemperature and curing time. Curing agents typically includecyanoguanidine, aromatic and aliphatic amines, acid anhydrides, LewisAcids, substituted ureas, imidazoles and hydrazines. The particularamount of curing agent required for each particular situation may bedetermined by well-established routine experimentation. Exemplarypreferred curing agents include imidazole(1,3-diaza-2,4-cyclopentadiene) available from Sigma Aldrich (St. Louis,Mo.) 2-ethyl-4-methylimidazole available from Sigma Aldrich and borontrifluoride amine complexes, such as Anchor 1170, available from AirProducts & Chemicals, Inc. (Allentown, Pa.).

The curing component, if desired, may include a carrier for the curingagent. The curing agents are suspended or dissolved in the carrier withthe resulting mixture applied to the porous body or other surface wheremixing with the resin will eventually take place. Exemplary carriersinclude ketones, such as acetone or methylethylketone or low molecularweight polyethyleneglycol. The amount of curing agent added to thecarrier and the carrier type will vary depending upon where the curingagent is to be located and the physical form of the curative oncedeposited. When the curing agent is to be located in a porous body, suchas a fiber tow, the carrier must be sufficiently liquid to penetrateinto the tow. In situations where the curing component is in the form ofa film, the carrier must be sufficiently viscous to form such films. Thecarrier can remain associated with the curative, for example in the caseof BF3 complexes hosted in polyethyleneglycol, the curative formulationbeing designed such that the presence of the carrier does not interferenegatively with the cured performance of the assembly. Alternatively,the carrier can simply be used as a method for impregnation of thecurative on the fabric where the carrier would typically be a highervolatility solvent such as the aforementioned methylethylketone oracetone, that is removed in a subsequent processing step leaving thereactive curative isolated on the porous body. Particularly exemplary ofthis mode of operation is the deposition of reactive solid imidazolecuratives on porous bodies from acetone.

FIG. 2 is a diagrammatic representation of a prepreg 30 in which fibertows 32 of the type shown in FIG. 1 are woven into a fabric 34 that iscoated on either side with resin layers 36 and 38. The woven fabric 34may be viewed as a compound porous body that has discontinuous outersurfaces on either side of the fabric and interstitial spaces 42. Inaddition, each of the fiber tows 32 forms a porous body as describedpreviously. Accordingly, there are porous bodies (tows) located withinthe porous body (fabric). The present invention is especially wellsuited for use in prepreg because the large reduction in viscosity thatis obtained when going from the high viscosity state to the lowviscosity state provides relatively complete and rapid penetration ofboth the overall fabric and the individual tows. Since the curing agents(+'s) are located in the tows, it is necessary that the resin completelypenetrate into the tows to provide uniform curing. The tows 32 in FIG. 2are shown having optional removable barriers 44 surrounding each tow. Itshould be noted that a single fabric layer 34 is shown sandwichedbetween two resin layers 36 and 38. In alternative embodiments, multiplefabric layers can be used where not all of the layers include thecurative component. For example, the fabric layer 34 can be sandwichedbetween two additional fabric layers that do not include a curativecomponent. The resin layers on the outside of the structure, whenconverted to the low viscosity state, would then penetrate through theadditional layers before contacting the curative component.

Another exemplary prepreg is shown generally at 50 in FIG. 3. Thisprepreg is similar to the prepreg shown in FIG. 2 in that it includesfiber tows 52 that have been woven into a single layer of fabric 54 thatis sandwiched between two layers of resin 56 and 58. The prepreg isshown at ambient temperature. At this temperature the resin component isin the high viscosity state which is sufficient to prevent a substantialamount of the resin component from flowing into or out of theinterstitial spaces 60 and/or away from the exterior surfaces of thefabric 54. In this embodiment, the curative component (+'s) is locatedwithin the interstitial spaces of the fabric and not limited to theinterstitial spaces of the tows. Optional removable reaction barriersare shown at 62 and 64. The removable reaction barrier is preferable insituations where it is desirable to eliminate any possible contactbetween the uncured resin and curing agent that might occur at theinterface between the two components.

Four exemplary embodiments of prepreg made using the resin component ofthe present invention are shown in FIG. 4. The prepreg 410 shown in FIG.4A has a resin component in the form of resin layer 412, a curativecomponent in the form of curing agent layer 414 and a porous body in theform of dry fabric layer 416. During storage at ambient temperature, theresin remains in a high viscosity state and does not flow to anysubstantial degree into the fabric layer 416. In accordance with thepresent invention, when the prepreg 410 is heated under pressure tocuring temperature, the resin is converted to the low viscosity stateand flows through the fabric layer 416 and into contact with thecurative component 414.

The prepreg shown at 420 in FIG. 4B is similar to the prepreg 410 exceptthat it includes two fabric layers 426 and 428 as well as the resinlayer 422 and curing agent layer 424. In accordance with the presentinvention, the resin component in the low viscosity state issufficiently fluid that is mixes thoroughly with the curing agent andalso penetrates both fabric layers. Another prepreg configuration isshown at 430 in FIG. 4C. The prepreg 430 includes resin layer 432 andcuring agent layer 434 that are separated by and sandwiched betweenfabric layers 436, 438 and 439. The resin becomes sufficiently fluid inthe low viscosity state to flow throughout the structure to provideuniform mixing of resin and curing agent as well as uniform penetrationof all three fabric layers. Prepreg 440 shown in FIG. 4D includes resinlayer 442, curing agent layer 444 and fabric layers 446 and 448. Theprepregs shown in FIG. 4 are all similar in that both the resincomponent and curative component are in the form of thin films that areseparated from each other by at least one layer of fabric.

Three more embodiments of prepreg in accordance with the presentinvention are shown diagrammatically in FIGS. 5A-5C. Prepreg 510 (FIG.5A) includes a resin component 512 that is composed of a fabric layerthat has been impregnated with an excess amount of uncured resin. Itshould be noted that only the layer 512 has an excess amount of uncuredresin. The overall assembly of 510 continues to have the usual “full”amount of resin which is typically around 35% by weight. The curativecomponent 514 is in the form of a layer of fabric that has beenimpregnated with the curing agent. The layer of resin-impregnated fabric512 is separated from the layer of curing agent impregnated fabric 514by a dry layer of fabric 518. An additional layer of dry fabric 516 mayoptionally be added to the prepreg, if desired. Such fabric that hasbeen impregnated with curing agent is disclosed in French Patentapplication No. 0210769 and co-pending U.S. patent application Ser. No.10/648,159. The contents of these applications are also specificallyincorporated by reference.

In FIG. 5B, a prepreg 520 is shown where the resin component is in theform of resin film 522 that is separated from “hardenerpreg” layer 524by a removable reaction barrier film 526. “Hardenerpreg” is a term ofart used to describe fiber or other types of reinforcement material thathas the curative dispersed in, and/or on the reinforcement and whereinthe reinforcement is free of resin. The prepreg 530 in FIG. 5C includesa layer of fabric 532 that has been impregnated with excess resin and ahardenerpreg layer 534. The hardenerpreg 534 and resin-impregnatedfabric 532 are separated by a removable reaction barrier film 536. Anadditional dry fabric layer 538 may be placed on top of the saturatedresin impregnated layer 532, if desired.

An uncured resin composition in accordance with the present invention isshown at 540 in FIG. 5D. The resin composition is intended for use as anadhesive or in other situations where the uncured resin is not stored aspart of a prepreg or other porous body. The composition 540 includes aresin layer 542 and a curing agent layer 544. A removable reactionbarrier 546 is used to separate two layers 542 and 544 from each other

A common feature in all of the above-described embodiments is that theuncured resin must change from a high viscosity state to a low viscositystate over a relatively small increase in temperature above ambienttemperature. This change is necessary to ensure uniform mixing andpenetration during the curing process. FIG. 6. is a graph showing therelationship between the viscosity and temperature of a given resin. Theviscosity range of the resin in the high viscosity state is shown by thedotted lines labeled “HIGH”. This is the viscosity range that isrequired to allow room temperature (ambient) handling and storage of theresin. The viscosity range of the resin in the low viscosity state isshown by the dotted lines labeled “LOW”. This is the viscosity rangerequired for adequate infusion of the resin into fabric layers and otherporous bodies during curing.

Uncured resins in accordance with the present invention have a viscosityprofile as shown at A in FIG. 6. As can be seen, the resin remains inthe high viscosity state throughout the ambient temperature range. Attemperatures slightly above the upper limit of ambient temperature(30°), the viscosity drops from the high viscosity state to the lowviscosity state. This is to be contrasted with resins loaded withparticles that have the viscosity profile shown at B. Theseparticle-loaded resins have a sharp viscosity transition at temperaturesin the same range as the present invention. However, as can be seen fromFIG. 6, the viscosity of these resins at ambient temperatures is toohigh to provide adequate prepreg handling characteristics. The viscosityprofiles of two non-modified or non-loaded conventional thermoset resinsare shown at C and D. Neither of these resins exhibit the sharptransition from a high viscosity state to a low viscosity state that isrequired by the present invention as shown at A. Example D can be seento demonstrate an acceptably low viscosity at a suggested curingtemperature of 60° C., but this is inseparable from it possessing aviscosity too high at ambient storage temperature (this manifestingitself particularly as a tack level that is too low). Example C on theother hand has an acceptable viscosity and tack at ambient temperature,but the form of its viscosity profile as a function of temperature meansthat the viscosity does not fall low enough for efficient impregnationat the curing temperature.

Examples of practice are as follows:

EXAMPLE 1

An uncured resin component was prepared by first combining DER 337 resinavailable form Dow Chemical Company (Wilmington, Del.) with didecyletherof 4,4′ biphenol as the gelation agent to form a thermosetting resinthat contains 5% by weight gelation agent. The mixture was heated to100° C. for a sufficient time to dissolve the gelation agent. Theresulting mixture was cooled to 30° C. One part of the resin mixture wasthen combined with two parts of crystallized Ruetapox 0158 particlesthat are available from Bakelite AG (Duisberg, Germany). The upper sizelimit of the particles was 2 mm. The resulting resin component can beformed into stable films at room temperature that have acceptable tackand handling properties for use in forming prepreg structures. Theviscosity of the resin component at room temperature (25° C.) issufficiently high to provide the above described acceptable tack andhandling properties. When the resin is heated to 60° C., the viscositydropped significantly to 0.2 Pas.

EXAMPLE 2

A resin component was made in the same manner as EXAMPLE 1 except thatthe upper size limit of the Ruetapox 0158 particles was 0.3 mm. Theviscosity of the resulting resin at room temperature was 57 Pas. Whenheated to 60° C., the viscosity of the resin dropped to 0.2 Pas. Whenusing solid particles that are relatively small (less than 1 mm), it ispreferred that the particles be combined with the resin component bysprinkling the solid particles onto a first film of the resin component.A second film of resin component is then applied over the top tosandwich the particles between the two resin films. The two resin filmscan be made from the same or different resins.

EXAMPLE 3

DER 337 resin (parts by weight) is mixed with Epikote 1001 (1 part byweight) which is available from Resolution Performance Products(Houston, Tex.). The resulting resin mixture is mixed with didecyl etherof 4,4′-biphenol as the gelation agent and heated to 100° C. to dissolvethe gelation agent. Sufficient gelation agent is added to provide aresin mixture that contains 5% by weight of the agent. After cooling to30° C., 1 part by weight of the resin mixture is combined with 2 partsof crystallized Ruetapox 0158 particles. The upper size limit of theRuetapox 0158 particles is 0.3 mm. The viscosity of the resulting resinat room temperature is 56 Pas. When heated to 60° C., the viscositydrops to 0.2 Pas. The viscosity of the resin increases to 15 Pas when itis cooled back down to room temperature.

EXAMPLE 4

A 1 mm thick resin film according to Example 1 was combined with layersof an 8H stain weave glass fabric, such as commercially available 7781glass from Hexcel Fabrics (Les Avenieres, France). The fabric wasinitially coated with Anchor 1170 to give an equivalent concentration of10 parts by weight Anchor 1170 for every 100 parts by weight of theresin film. The part was then placed in a vacuum bag for 1 hour at 60°C. to form the fully impregnated cured laminate. The T_(g) of the curedlaminate was a single transition measured as 58° C. (onset E′, DMA)which indicates homogeneous curing.

Having thus described exemplary embodiments of the present invention, itshould be noted by those skilled in the art that the within disclosuresare exemplary only and that various other alternatives, adaptations andmodifications may be made within the scope of the present invention.Accordingly, the present invention is not limited to the above preferredembodiments and examples, but is only limited by the following claims.

What is claimed is:
 1. A method for making a composite material that issuitable for storage at ambient temperatures, said method comprising thesteps of: providing a porous body comprising an exterior surface andinterior surfaces located within said porous body, said interiorsurfaces further defining interstitial spaces located within said porousbody; providing an uncured resin component that forms a cured resin whenexposed to a curing agent and heated to a resin component curingtemperature that is higher than said ambient temperature, said uncuredresin component comprising a liquid part including one or more liquidthermosetting resins and a solid part that comprises particles of one ormore solid thermosetting resins wherein said solid part is dispersedwithin said liquid part, and wherein said particles have a melting pointthat is above said ambient temperature and below said resin componentcuring temperature, said uncured resin component further comprising agelation agent that is present in a sufficient amount to form saidliquid part into a gel that is sufficiently gelatinous to maintain saidparticles in suspension within said liquid part at said ambienttemperatures, said gelation agent having a melting temperature that isbelow said resin component curing temperature and wherein the viscosityof said uncured resin component changes from a high viscosity state to alow viscosity state when the temperature of said uncured resin componentis increased from said ambient temperature to said resin componentcuring temperature and wherein said high viscosity state issubstantially more viscous than said low viscosity state; locating saiduncured resin component at the exterior surface of said porous body,said uncured resin component being in said high viscosity state suchthat said uncured resin does not flow into said interstitial spaces atsaid ambient temperatures; providing a curative component that comprisesa curing agent for said uncured resin component; and locating saidcurative component at the interior surfaces of said porous body, saidcurative component being located such that said curative componentremains separated from said uncured resin component while said uncuredresin component remains in said high viscosity state at said exteriorsurface during storage of said composite material at said ambienttemperature.
 2. A method for making a composite material according toclaim 1 wherein said porous body comprises a fiber component comprisinga plurality of fibers wherein each of said fibers comprises a fibersurface, said fibers being oriented to provide said porous bodycomprising an exterior surface defined by said fiber surfaces locted atthe exterior surface of said porous body, said porous body furthercomprising interior surfaces defined by said fiber surfaces locatedwithin said porous body.
 3. A method for making a composite materialaccording to claim 2 wherein said fiber component comprises a fiberbundle.
 4. A method for making a composite material according to claim 3wherein said fiber component comprises a plurality of fiber bundles. 5.A method for making a composite material according to claim 4 whereinsaid plurality of fiber bundles are in the form of a fabric orunidirectional tape.
 6. A method for making a composite materialaccording to claim 5 wherein said plurality of fibers form a fabric orunidirectional tape having a first side and a second side that definethe exterior surface of said porous body and wherein said uncured resincomponent is located at both the first side and second side of saidfabric or unidirectional tape and said curative component is located atthe interior surfaces of said fabric or unidirectional tape.
 7. A methodfor making a composite material according to claim 1 wherein a removablereaction barrier located between said uncured resin component and saidcurative component to prevent contact of said uncured resin componentand said curative component during storage of said composite material atambient temperatures.
 8. A method for making a composite materialaccording to claim 7 wherein said uncured resin component is locatedonly at the exterior surface of said porous body and said removablereaction barrier is located between the exterior surface of said porousbody and said uncured resin component.
 9. A method for making acomposite material according to claim 8 wherein said porous body is inthe form of a woven fabric.
 10. A method for making a composite materialcomposition according to claim 9 where said woven fabric has a firstside and a second side that define the exterior surface of said porousbody and wherein said uncured resin component is located only at theexterior surface of said woven fabric and said removable reactionbarrier is located between the exterior surface of said porous body andsaid uncured resin component.
 11. A method for making a compositematerial according to claim 1 wherein the difference in viscositybetween said high viscosity state and said low viscosity state is atleast 10 Pas.
 12. A method for making a composite material according toclaim 1 wherein the difference between said ambient temperature and saidcuring temperature is between 10° C. and 60° C.
 13. A method for makinga composite material according to claim 12 wherein the difference inviscosity between said high viscosity state and said low viscosity stateis at least 10 Pas.
 14. A method for making a composite materialaccording to claim 1 wherein the viscosity of said uncured resincomponent in said high viscosity state is between 20 Pas and 70 Pas andwherein the viscosity of said uncured resin component in said lowviscosity state is between 0.1 Pas and 10 Pas.
 15. A method according toclaim 1 which includes the additional step of curing said uncured resincomponent after said composite material has been made.
 16. A methodaccording to claim 15 wherein said uncured resin component is cured at atemperature of between 40° C. and 90° C.
 17. A method according to claim1 that includes the additional step of storing said composite materialat room temperature.